1. Field of the Invention
The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space provided with a filling of mercury and an inert gas in a gastight manner,
which discharge vessel contains an amalgam which communicates with the discharge space,
and the low-pressure mercury-vapor discharge lamp comprises discharge means for maintaining an electric discharge in the discharge space.
The invention also relates to an amalgam for use in the low-pressure mercury-vapor discharge lamp.
2. Discussion of the Prior Art
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner wall of the discharge vessel may be coated with a luminescent layer comprising a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, such as UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general lighting purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of low-pressure mercury-vapor discharge lamps is generally tubular and circular in section, and comprises both elongated and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of comparatively short, straight parts having a comparatively small diameter, which straight parts are interconnected, on the one hand, by means of bridge parts or, on the other hand, by means of, for example, arc-shaped parts. Compact fluorescent lamps are generally provided with a lamp cap (with integrated electronics).
In the description and the claims of the current invention, the designation xe2x80x9cnominal operationxe2x80x9d is used to refer to operating conditions where the mercury-vapor pressure is such that the radiation output of the lamp is at least 80% of that during optimum operation, i.e. under operating conditions where the mercury-vapor pressure is optimal. The amalgam limits the mercury-vapor pressure in the discharge vessel with respect to a discharge lamp containing only free mercury. This enables nominal operation of the lamp at comparatively high lamp temperatures, which may occur, for example, when the lamp is subjected to a high load or when the lamp is used in a closed or badly ventilated luminaire. Furthermore, in the description and the claims, the xe2x80x9cinitial radiation outputxe2x80x9d is defined as the radiation output of the discharge lamp 1 second after switching on the discharge lamp, and the xe2x80x9crun-up timexe2x80x9d is defined as the time needed by the discharge lamp to reach a radiation output of 80% of that during optimum operation.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, also referred to as a vapor pressure-controlled lamp, is disclosed in U.S. Pat. No. 4,093,889. The known lamp has a comparatively low mercury-vapor pressure at room temperature. As a result, the known lamp has the disadvantage that also the initial radiation output is comparatively low when a customary power supply is used to operate said lamp. In addition, the run-up time is comparatively long because the mercury-vapor pressure increases only slowly after switching on the lamp.
Apart from the above-described amalgam lamps, low-pressure mercury-vapor discharge lamps are known which comprise both a (main) amalgam and a so-called auxiliary amalgam. If the auxiliary amalgam comprises sufficient mercury, then the lamp has a relatively short run-up time. Immediately after the lamp has been switched on, i.e. during preheating the electrodes, the auxiliary amalgam is heated by the electrode so that it relatively rapidly dispenses a substantial part of the mercury that it contains. In this respect, it is desirable that, prior to being switched on, the lamp has been idle for a sufficiently long time to allow the auxiliary amalgam to take up sufficient mercury. If the lamp has been idle for a comparatively short period of time, the reduction of the run-up time is only small. In addition, in that case the initial radiation output is (even) lower than that of a lamp comprising only a main amalgam, which can be attributed to the fact that a comparatively low mercury-vapor pressure is adjusted in the discharge space by the auxiliary amalgam. An additional problem encountered with comparatively long lamps is that it takes comparatively much time for the mercury liberated by the auxiliary amalgam to spread throughout the discharge vessel, so that after switching on such lamps, they demonstrate a comparatively bright zone near the auxiliary amalgam and a comparatively dark zone at a greater distance from the auxiliary amalgam, which zones disappear after a few minutes.
Furthermore, low-pressure mercury-vapor discharge lamps are known which are not provided with an amalgam and contain only free mercury. These lamps, also referred to as mercury lamps, have the advantage that the mercury-vapor pressure at room temperature and hence the initial radiation output are comparatively high. In addition, the run-up time is comparatively short. After having been switched on, comparatively long lamps of this type also demonstrate a substantially constant brightness over substantially the whole length, which can be attributed to the fact that the vapor pressure (at room temperature) is sufficiently high at the time of switching on these lamps. Nominal operation at comparatively high lamp temperatures can be achieved using a mercury lamp whose discharge space contains (just) enough mercury to bring about a mercury-vapor pressure at the operating temperature which is close to the optimum mercury-vapor pressure. During the service life of the lamp, however, mercury is lost because it is bound, for example, to a wall of the discharge vessel and/or to emitter material. As a result, in practice such a lamp only has a limited service life. Therefore, the mercury dose in mercury lamps is substantially higher, in practice, than the quantity of mercury necessary during nominal operation in the vapor phase. However, this has the disadvantage that the mercury-vapor pressure is equal to the saturation vapor pressure pertaining to the temperature of the coldest spot of the discharge vessel. As the saturation vapor pressure increases exponentially with temperature, temperature variations, occurring for example in a badly ventilated luminaire or when the lamp is subjected to a high load, lead to a reduction of the radiation output. At comparatively low ambient temperatures, the mercury-vapor pressure decreases, which also leads to a reduction of the radiation output.
It is an object of the invention to provide a lamp of the type described in the opening paragraph, which, when it is used regularly, has a comparatively high initial radiation output and a comparatively short run-up time as well as a comparatively high radiation output in a comparatively large ambient-temperature range.
This object is achieved in accordance with the invention in that the amalgam comprises a bismuth-lead compound having a lead content (Pb) in the range between 35xe2x89xa6Pbxe2x89xa660 at. %, a bismuth content (Bi) in the range between 40xe2x89xa6Bixe2x89xa665 at. %, and a mercury content (Hg) in the range between 0.05xe2x89xa6Hgxe2x89xa61 at. %.
The advantage of using such a Bixe2x80x94Pb amalgam is that, at room temperature, the mercury-vapor pressure is comparatively close to that of liquid mercury. If the amalgam has the above-mentioned composition, the discharge lamp is nominally operated at a corresponding coldest spot temperature of the discharge vessel which lies in a comparatively wide temperature range from 65 to 165xc2x0 C. A further advantage of the use of such a Bixe2x80x94Pb amalgam resides in that the curves, in which the mercury-vapor pressure is plotted as a function of the temperature, can be adjusted via the mercury content. Said properties of the (main) amalgam, i.e. the wide temperature interval and the variable mercury-vapor pressure curves, are obtained by the choice of the composition of the Bixe2x80x94Pb amalgam in accordance with the invention.
A further advantage of the use of a Bixe2x80x94Pb amalgam in accordance with the invention resides in that the amalgam can be used in low-pressure mercury-vapor discharge lamps which can be dimmed.
Preferably, the lead content in the amalgam lies in the range between 40xe2x89xa6Pbxe2x89xa650 at. %, and the bismuth content lies in the range between 50xe2x89xa6Bixe2x89xa660 at. %. Particularly suitable are compositions of the amalgam near the Bixe2x80x94Pb eutectic point at 44 at. % Pb.
The above-mentioned composition of the Bixe2x80x94Pb amalgam enables, in operation, at least 80% of the radiation output (nominal operation) of the low-pressure mercury-vapor discharge lamp to be achieved at a corresponding temperature of the coldest spot of the discharge vessel which lies in a relatively wide temperature range from 65 to 165xc2x0 C. The run-up time of the discharge lamp comprising a Bixe2x80x94Pb amalgam in accordance with the invention is less than ten minutes, in either case, while an auxiliary amalgam reduces the run-up time to less than 3 minutes. Amalgams of a composition in accordance with the invention are particularly suitable for use in (energy-saving) (compact) low-pressure mercury-vapor discharge lamps. Such discharge lamps have a good initial radiation output and combine a comparatively short run-up time with, at nominal operation, a comparatively wide interval for the temperature of the coldest spot of the discharge vessel. As a result, nominal lamp operation is possible in a comparatively large temperature interval.
Preferably, the mercury content (Hg) lies in the range between 0.05 and 0.75 at. % Hg.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the amalgam further comprises gold, the gold content (Au) lying in the range between 0.1xe2x89xa6Auxe2x89xa620 at. %.
Using the above-mentioned composition of the Bixe2x80x94Pbxe2x80x94Au amalgam, in operation, at least 80% of the radiation output (nominal operation) of the low-pressure mercury-vapor discharge lamp is achieved at a corresponding temperature of the coldest spot of the discharge vessel which lies in a relatively wide temperature range from 50 to 160xc2x0 C., while at least 90% of the radiation output is achieved at a corresponding temperature of the coldest spot which lies in a relatively wide temperature range from 70 to 130xc2x0 C.
An additional advantage of the use of such a Bixe2x80x94Pbxe2x80x94Au amalgam is that the curves, in which the mercury-vapor pressure is plotted as a function of the temperature, cannot only be adjusted via the mercury content but also via the composition of the amalgam.
The compositions of said Bixe2x80x94Pbxe2x80x94Au amalgams in accordance with the invention are chosen to be such that the amalgam melts in a temperature range from 100 to 140xc2x0 C. In addition, the small mercury content of said amalgams brings about a comparatively low mercury activity at higher temperatures (140-175xc2x0 C.), the amalgam being present in the liquid state in the discharge vessel (the mercury is in the vapor phase). A comparatively high mercury activity at comparatively low temperatures is obtained in that the mercury does not readily mix with the underlying alloys. Bixe2x80x94Pbxe2x80x94Au amalgam compositions are particularly suitable, in which the gold is added close to the above-mentioned eutectic point of Bi and Pb. Such amalgams have a Bi:Pb ratio of 56:44.
Preferably, the gold content in the amalgam lies in the range between 8xe2x89xa6Auxe2x89xa612 at. %. Bixe2x80x94Pbxe2x80x94Au amalgams of such a composition exhibit a double peak in the mercury-vapor-pressure curves, which is caused by the melting of a large quantity of the ternary intermetallic compound of the structural formula BiPb3Au above the Bixe2x80x94Pb eutectic point (at 125xc2x0 C.).
A further advantage of the addition of gold to Bixe2x80x94Pb amalgams is that, at low temperatures (room temperature), the mercury-vapor pressure is substantially independent of the mercury concentration up to very low mercury concentrations (0.3% Hg). As a result, the discharge lamp is comparatively insensitive to (irreversible) mercury loss in other lamp components, for example at the wall of the discharge vessel and/or at emitter material.
Apart from the above-mentioned materials, the amalgam in accordance with the invention may comprise additions of, for example, zinc, silver, gallium, indium, tin, antimony and/or other elements. It is desirable that such additions do not move the melting temperature range (100-140xc2x0 C.) of the Bixe2x80x94Pb alloys by more than 20xc2x0 C.
At the start of the service life of a low-pressure mercury-vapor discharge lamp, comparatively much mercury can be bound at the wall during operation. To preclude this, the discharge vessel of a lamp in accordance with the invention may be coated with a metal-oxide protective layer at an inner surface. Such a protective layer, for example of scandium oxide, yttrium oxide, lanthanum oxide or an oxide of one of the lanthanide""s, counteracts the loss of mercury caused by binding at the wall. A discharge lamp with a small mercury consumption is favorable since it enables a more optimum design of the amalgam.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.